Stiffening of portions of shoe uppers by coating the portion to be stiffened with a layer of molten resin and solidifying the resin was disclosed in the U.S. Pat. to Chaplick et al. No. 3,316,573. In the process of that patent, molten thermoplastic polymeric material was spread as a layer on a shoe upper at a temperature at which the thermoplastic material has a viscosity low enough to wet and adhere to the surface of the article to be stiffened but sufficiently high so that it will substantially not penetrate the shoe component, and the layer of material so formed was cooled and shaped to form a stiff, resilient layer holding the article in the desired configuration.
The process referred to has entered into substantial commercial use for the stiffening of toe portions of shoes; but heel portions of shoes because of the relatively small radius curves and the seam, have not lent themselves to deposition of a useful thermoplastic material stiffening layer by procedures shown in the patent.
Stiffening of portions of shoes by disposing them between mating mold halves and injecting molten stiffener material between the shoe upper layers in the mold has been proposed. However, this method has severe economic and operational difficulties which have prevented its acceptance. That is, the special two-part mold is costly particularly since different molds would be required for every distinct style of heel end and in some cases different molds would be required for different sizes of the same style. Further disadvantages are that since molten resin is introduced between two layers of shoe material, dissipation of heat to allow stiffening of the resin is retarded by the heat insulating action of the shoe upper materials so that the "in mold" time is long. Also, the thickness of the resin layer formed is dependent on the space left in the mold between the layer of upper material and the liner and hence varies inversely as the thicknesses of the upper material and liner which in the case of leather are known to be highly variable even between the two sides of a shoe.
A further procedure for stiffening portions of shoes involves the disposition of a thermosetting material on a stiffener blank or on a portion of the upper by means of a special distributor head. This distributor head squeezes out a pattern of a pasty material from a series of extruder orifices in a plate pressed against the blank or shoe upper and this pattern is spread as a uniform layer by applying pressure to flow the material over the selected area of the shoe component. Because of "stringing" and other problems this procedure is not suitable for application of molten resinous stiffeners.